Despite the enormous scientific effort devoted in the last two decades to developing pharmacological approaches for neurodegenerative diseases, very few effective drugs have been discovered. Thus, there are no effective therapies for most of these devastating disorders. A clear example is provided by Alzheimer's disease (AD) Alzheimer's disease is the most common cause of memory loss in the elderly population, and the 5th leading cause of death in this group, currently affecting almost 50 million individuals worldwide. The number will increase dramatically in the coming decades as the population ages, producing devastating medical and socio-economic consequences. Although some of the basic molecular mechanisms underlying AD have been identified, this has not resulted in effective treatments for this devastating disorder. The disease is associated with accumulation in the brain of the 40-42 amino acid amyloid-β (Aβ) peptide, a cleavage product of the amyloid precursor protein (APP). Aβ spontaneously forms polymers ranging from small, soluble oligomers to large, insoluble fibrils. Compelling evidence suggests that soluble Aβ oligomers, rather than fibrillar aggregates, are primarily responsible for the synaptic dysfunction underlying the cognitive decline in Alzheimer's disease. Aβ oligomers are believed to act by binding to cell surface receptors that transduce their detrimental effects on synapses. However, the identity of these receptors remains uncertain. The identification of neuronal receptor sites for Aβ oligomers has important therapeutic implications, since these represent potential targets for pharmacological intervention. Drugs that block Aβ binding to its neuronal receptors and inhibit downstream neurotoxic effects may offer a significant advantage over current therapies, since such compounds target the earliest molecular abnormalities in synaptic function.
The two major neuronal systems previously targeted for preventing this disease are the cholinergic and the glutamatergic systems. Currently approved treatments for Alzheimer's disease, including cholinesterase inhibitors such as Donepezil (Aricept) and the NMDA receptor antagonist memantine, offer temporary symptomatic relief, but do not significantly delay the course of the disease. The reason for the lack of efficacy of these compounds is likely due the fact that their targets are not involved in the earliest alterations in synaptic function that initiate the pathogenic process.
Following the amyloid cascade hypothesis, additional targets have been considered in recent years. Alternative strategies have included: (i) decreasing production of the Aβ peptide by inhibition of β- and γ-secretases, or stimulation of the α-secretases (the cleaving enzymes responsible for the processing of the APP protein); (ii) increasing Aβ clearance by either active (vaccination) or passive (monoclonal antibodies) immunization, up-regulation of degrading enzymes (Neprelysin and the insulin degrading enzyme), or stimulation of Aβ transport out of the brain (by altering the RAGE/LRP-1 pathway). Unfortunately, however, none of these approaches (some of which have reached phase III clinical trials) has shown significant effects in preventing the cognitive decline in Alzheimer's patients.
Recently, a novel and surprising candidate has emerged as a receptor for Aβ oligomers: the cellular form of the prion protein (PrPC), a membrane glycoprotein expressed on the neuronal surface, PrPC. PrPC is an endogenous, cell-surface glycoprotein, plays an important role in transmissible neurodegenerative disorders such as Creutzfeldt-Jakob disease and bovine spongiform encephalopathy (commonly referred to as prion diseases), by serving as the substrate for formation of PrPSc, the infectious form of PrP. It has been previously reported that Aβ oligomers (but not monomers or fibrils) bind with low nanomolar affinity to PrPC via two sites within the unstructured N-terminal tail (residues 23-27 and 95-105). Binding was not observed with Aβ monomers or fibrils, suggesting that PrPC is specifically a receptor for oligomers. Importantly, PrPC was also found to be a mediator of Aβ-induced synaptotoxicity. As a consequence of this interaction, PrPC transduces the synaptotoxic effects of Aβ oligomers via intracellular signaling cascades. Although some studies have challenged this model, others have shown that PrPC could be targeted to block Aβ in vivo. For example, application of anti-PrP antibodies, or genetic ablation of PrPC, can prevent Aβ-induced synaptic dysfunction in hippocampal slices or transgenic mice. In particular, hippocampal slices derived from PrP null mice were found to be resistant to Aβ oligomer-induced suppression of long-term potentiation (LTP), an in vitro correlate of memory and synaptic function. Additionally, PrPC was required for both the cognitive deficits and reduced survival observed in transgenic mouse models of Alzheimer's disease. The identification of PrPC as a cell-surface receptor that mediates the neurotoxic effects of the Aβ oligomers therefore suggests that alterations in the normal function of PrPC could play a role in the pathogenesis of Alzheimer's diseases (see FIGS. 1A and 1C). Thus, there is a need to identify molecules and inhibitors to block Aβ oligomer binding to PrPC as a treatment for AD
Prion Diseases and PrPC. Amazingly, PrPC has been studied for the last two decades in the context of a different group of neurodegenerative disorders, known as prion diseases. Prion diseases, including bovine spongiform encephalopathy (“mad cow disease”) and its human counterparts, are rare neurodegenerative disorders caused by an unusual type of infectious agent (prion) that consist of a self-propagating protein molecule. Prion diseases are caused by conversion of PrPC, a normal cell-surface glycoprotein, into PrPSc, a conformationally altered isoform that serves as a molecular template for generation of additional molecules of PrPSc. While much research has focused on characterizing the mechanisms of formation and replication of prions, little progress has been made in defining the neurodegenerative pathways operative in prion diseases. Recent evidence indicates that the toxicity of PrPSc requires the presence of membrane-anchored PrPC at the cell surface, and suggests that the normal, physiological activity of PrPC is subverted to produce toxicity (see FIGS. 1B and 1D). Previous attempts to treat prion diseases by simply lowering the load of PrPSc have been largely unsuccessful. This may be due to the fact that PrPC-mediated neurotoxicity, once unleashed, remains active even if formation of PrPSc is inhibited. Therefore, the most effective anti-prion treatments could be those that block prion-induced toxic pathways, as well as PrPSc formation.
The physiological activity of PrPC has so far remained elusive. In an attempt to provide insights into the normal activity of PrPC, the inventors recently demonstrated that deletions in the conserved central region endow the protein with a highly toxic activity that is likely related to its normal function. In particular, the inventors found that mutations in the central region of PrP (residues 105-125), including artificial deletions as well as point mutations associated with familial prion diseases of humans, induce a powerful ion channel activity at the plasma membrane that can be detected in transfected cells by patch-clamping techniques. This activity is dose-dependently suppressed by co-expression of wild-type PrP, indicating that it is related to a normal physiological activity of PrPC.
Accordingly, PrPC could act as a receptor not only for PrPSc and Aβ oligomers, but also for other β-rich proteins. Amazingly, the same two sites involved in binding of PrPC to Aβ oligomers and other aggregated proteins (residues 23-28 and 95-105) also determine the ion channel activity of mutant PrP, which could contribute to protein aggregate-mediated increased ion channel activity in PrPC similar to that caused by mutant PrP. Therefore, a PrP-dependent ion channel activity may contribute to the pathogenesis of prion disease, Alzheimer's disease, and several other neurodegenerative disorders.
Accordingly, there is a need in the art for identification of small molecule ligands for PrPC that prevent binding of toxic protein aggregates and providing a new therapeutic strategy for treatment of prion and Alzheimer's diseases, as well as other neurodegenerative disorders due to protein aggregation.